A STIS Survey for O VI Absorption Systems at 0.12 < z lesssim 0.5. I. The Statistical Properties of Ionized Gas

Abstract

We have conducted a systematic survey for intervening O VI absorbers in available echelle spectra of 16 QSOs at z_QSO = 0.17-0.57. These spectra were obtained using HST STIS with the E140M grating. Our search uncovered a total of 27 foreground O VI absorbers with rest-frame absorption equivalent width W_r(1031) gtrsim 25 mÅ. Ten of these QSOs exhibit strong O VI absorbers in their vicinity. Our O VI survey does not require the known presence of Ly α , and the echelle resolution allows us to identify the O VI absorption doublet based on their common line centroid and known flux ratio. We estimate the total redshift survey path, Δ z, using a series of Monte Carlo simulations, and find that Δ z = 1.66,2.18, and 2.42 for absorbers of strength W_r = 30,50, and 80 mÅ, respectively, leading to a number density of dScript N(W >= 50 mÅ)/dz = 6.7 +/- 1.7 and dScript N(W >= 30 mÅ)/dz = 10.4 +/- 2.2. In contrast, we also measure dScript N/dz = 27 +/- 9 for O VI absorbers of W_r > 50 mÅ at | Δ v| < 5000 km s^-1 from the background QSOs. Using the random sample of O VI absorbers with well-characterized survey completeness, we estimate a mean cosmological mass density of the O VI gas Ω (O^{5 +}) h = (1.7 +/- 0.3) × 10^-7. In addition, we show that <5% of O VI absorbers originate in underdense regions that do not show a significant trace of H I. Furthermore, we show that the neutral gas column N(H I) associated with these O VI absorbers spans nearly 5 orders of magnitude, and shows moderate correlation with N(O VI). Finally, while the number density of O VI absorbers varies substantially from one sight line to another, it also appears to be inversely correlated with the number density of H I absorbers along individual lines of sight.

Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.